In the oil and gas exploration and production industry bores are drilled from surface to access subsurface hydrocarbon reservoirs. The bores are typically drilled in sections: a section of bore is drilled using drilling apparatus including a bit mounted on the end of a string; the drilling apparatus is pulled out of the bore; a section of bore-lining tubing is run into the “open” bore; and the tubing is then cemented or otherwise sealed in the bore by filling the annulus between the tubing and the bore wall with cement slurry. These drilling and lining steps are repeated until the bore is of the required length or depth. Clearly, this can be a time-consuming operation as to drill and line each bore section it is necessary to make up and then dismantle first a drill string and then a running string, both of which may be several thousand metres long.
Furthermore, with conventional bore-lining techniques, the outer diameter of each section of bore-lining tubing must be smaller than the inner diameter of the preceding tubing to enable the tubing to be run into the bore. Thus, a step-wise reduction in bore diameter occurs at the transition between each bore section. The reduction in available bore diameter limits the production capabilities of the well, restricts access to the well, and also requires the use of smaller diameter and thus generally less robust drilling apparatus in the lower portions of the well. A further disadvantage also arises in that the upper portions of the bore may have to be drilled to a relatively large diameter, in light of the numerous subsequent diameter reductions that must be accommodated, which increases drilling time and expense.
Some of these disadvantages may be overcome by the use of expandable bore lining tubing, which may be run in through a section of existing tubing and then expanded to a larger diameter. However, to preserve bore diameter it is important that the desired degree of overlap between adjacent sections is maintained. This may be problematic when, for example, it is not possible to run the tubing to the bottom of the bore. This may occur due to material gathering in the lower end of the bore, or the tubing encountering an unexpected bore diameter restriction.
Another proposal, as described in U.S. Pat. No. 6,457,532 and U.S. Ser. No. 09\469,643 the disclosures of which are incorporated herein by reference, is to form the lower end of the drill string of expandable tubing. Thus, if a problem formation is encountered in the course of a drilling operation, the tubing may be expanded without the delay that would be involved in pulling out the drilling apparatus and then running in and expanding a section of bore-lining tubing.
However, with this method, if a problem formation is encountered early in the drilling operation, only a short section of the expandable tubing is utilised to line open bore, and a significant portion of the tubing is located within the existing casing or liner and thus serves no useful purpose, and further restricts the available bore diameter. Alternatively, if no problems are encountered, the length of bore which can be lined is restricted by the length of the expandable tubing previously incorporated in the string.
Furthermore, the expandable tubing which forms the lower end of the drill string as proposed in PCT\GB99\04246 is likely to represent a compromise between the qualities and properties required to withstand the weight and torque which must be transmitted from surface via the tubing to the drill bit, to allow drilling fluid to be carried to the bit, to have sufficient abrasion resistance to avoid damage from contact with surrounding casing or bore wall, and to allow installation and expansion to create a safe and secure bore lining.